The present invention relates to a base station apparatus and a method for suppressing a peak power for a cellular system of an automobile telephone, a portable telephone and the like.
A cellular system for an automobile telephone, a portable telephone and the like, the demand of which is rapidly increasing in recent years, is a system where a base station is located at the center of each cell and the base station and one or more communication terminals in the cell simultaneously perform radio communication in multiple access.
A code division multiple access (CDMA) system, one of the multiple access systems, is a system where a signal spread in a wide band that is obtained by multiplying modulated information data by a spreading code is transmitted from the transmission side and the information data is demodulated on the reception side by performing the despreading of a received signal by multiplying it by the same spreading code as that on the transmission side at the same timing as that on the transmission side. Because each user can utilize the same frequency band in the CDMA system, the increase of channel capacity can be schemed. Consequently, the CDMA system is widely noticed for a cellular system.
However, each user utilizes the same frequency band at the same time in the CDMA system, a base station should transmit a plurality of signals by multiplexing them. Consequently, the CDMA system has a problem that a transmission amplitude at a peak time becomes very high in comparison with an average amplitude. For preventing the distortion of a transmission signal even at the time of a peak, an amplifier having a large linear operating range should be used. Then, a high voltage power source is needed, which makes the apparatus larger.
Accordingly, for overcoming the problem, methods for suppressing the transmission amplitude at a peak time have conventionally been examined. There is a conventional base station disclosed in Japanese laid-open patent publication Hei 10-126309 as a base station that can suppress the transmission amplitude at a peak time.
FIG. 1 is a block diagram showing the structure of a conventional base station. Incidentally, in the following description, a case where QPSK modulation is used in primary modulation will be described as an example. Furthermore, it is supposed that a base station is performing radio communication with three users A-C.
In FIG. 1, a modulation section 1 performs the QPSK modulation of a transmission signal A to be transmitted to a user A, and outputs an in-phase component and an orthogonal component of the signal after the modulation to a spreading section 4. Similarly, a modulation section 2 performs the QPSK modulation of a transmission signal B to be transmitted to a user B, and outputs an in-phase component and an orthogonal component of the signal after the modulation to a spreading section 5. A modulation section 3 performs the QPSK modulation of a transmission signal C to be transmitted to a user C, and outputs an in-phase component and an orthogonal component of the signal after the modulation to a spreading section 6.
The spreading section 4 performs spread processing in which a peculiar spreading code is multiplied to the transmission signal A modulated in the QPSK modulation, and outputs the spread signal to a multiplexing section 7. Similarly, the spreading section 5 performs spread processing in which a peculiar spreading code is multiplied to the transmission signal B modulated in the QPSK modulation, and outputs the spread signal to the multiplexing section 7. The spreading section 6 performs spread processing in which a peculiar spreading code is multiplied to the transmission signal C modulated in the QPSK modulation, and outputs the spread signal which has been performed by the filter 10, and when the measured value is larger than a previously set permissible amplitude value, the amplitude controlling section 13 controls the attenuation amount of the in-phase component of a transmission signal at an attenuation section 17. Similarly, the amplitude controlling section 14 measures the amplitude value of a signal, the band restriction of which has been performed by the filter 11, and when the measured value is larger than a previously set permissible amplitude value, the amplitude controlling section 14 controls the attenuation amount of the orthogonal component of a transmission signal at an attenuation section 18.
A delay section 15 delays a signal of an in-phase component outputted from the multiplexing section 7 for a time equal to a necessary time for a series of attenuation amount operating processing performed in the interpolation section 8, the filter 10 and the amplitude controlling section 13, and the delay section 15 outputs the delayed signal to the attenuation section 17. Similarly, a delay section 16 delays a signal of an orthogonal component outputted from the multiplexing section 7 for a time equal to a necessary time for a series of attenuation amount operating processing performed in the interpolation section 9, the filter 11 and the amplitude controlling section 14, and the delay section 16 outputs the delayed signal to the attenuation section to the multiplexing section 7.
The multiplexing section 7 divides spread signals outputted from the spreading sections 4-6 into in-phase components and orthogonal components to add them respectively. The multiplexing section 7 then outputs an in-phase component signal to an interpolation section 8 and a delay section 15, and outputs an orthogonal component signal to an interpolation section 9 and a delay section 14.
The interpolation sections 8 and 9 increase their sampling rates by M (M is a natural number) times, and perform zero-insertion interpolation where zero is inserted at a sampling point where no signal exists, respectively.
A filter 10 performs the band restriction of an interpolated signal outputted from the interpolation section 8 by means of a filter coefficient set in a filter coefficient memory 12 previously, and outputs the signal after the band restriction to an amplitude controlling section 13. Similarly, a filter 11 performs the band restriction of an interpolated signal outputted from the interpolation section 9 by means of a filter coefficient set in the filter coefficient memory 12 previously, and outputs the signal after the band restriction to an amplitude controlling section 14.
The amplitude controlling section 13 measures the amplitude value of a signal, the band restriction of 18.
The attenuation section 17 attenuates the amplitude of the in-phase component of a transmission signal under the control of the amplitude controlling section 13. Similarly, the attenuation section 18 attenuates the amplitude of the orthogonal component of a transmission signal under the control of the amplitude controlling section 14.
Interpolation sections 19 and 20 increase their sampling rates by M (M is a natural number) times, and perform zero-insertion interpolation where zero is inserted at a sampling point where no signal exists, respectively.
A filter 21 performs the band restriction of an interpolated signal outputted from the interpolation section 19 by means of a filter coefficient set in the filter coefficient memory 12 previously, and outputs the signal after the band restriction to a D/A conversion section 23. Similarly, a filter 22 performs the band restriction of an interpolated signal outputted from the interpolation section 20 by means of a filter coefficient set in the filter coefficient memory 12 previously, and outputs the signal after the band restriction to a D/A conversion section 24.
The D/A conversion section 23 converts a transmission signal of a digital in-phase component outputted from the filter 21 to an analog signal. Similarly, the D/A conversion section 24 converts a transmission signal of a digital orthogonal component outputted from the filter 22 to an analog signal.
As described above, the conventional base station measures an amplitude value of a signal the band of which is restricted by means of filter operations, and when the measured value is larger than a permissible amplitude value set previously, the transmission amplitude at the time of a peak is suppressed by controlling the attenuation amount of a transmission signal.
However, the filter operation needs a long tap length for preventing deterioration in its pass band and for obtaining very large suppressing characteristics in its rejection band. The aforesaid base station needs two filter operation circuits per component. For example, in case of the QPSK modulation, four filter operation circuits are needed.
That is, the aforesaid conventional base station should increase its filter operation circuit for suppressing a transmission amplitude at the time of a peak. Consequently, the conventional base station has a problem that the scale of its circuit increases and its consumption electric power also increases.
The object of the present invention is to provide a base station apparatus/capable of suppressing a transmission amplitude at the time of a peak without increasing the number of its filter operation circuits and a method for suppressing its peak power.
The object can be attained by calculating a correction coefficient when the amplitude of a transmission signal is beyond a permissible amplitude value, and by subtracting a correction value obtained by multiplying a correction coefficient by a filter coefficient from a transmission signal after filtering operation.